This invention relates to an ion storage device (alternatively termed an ion buncher) and it relates particularly, though not exclusively, to an ion storage device suitable for use in a time-of-flight mass spectrometry system.
In order that a time-of-flight mass spectrometry system may have an acceptable mass resolving power, ions should enter the flight path of the spectrometer in bursts of short duration, of typically 1 to 10 nsec. If, as is often the case, the ions are extracted from a continuous ion beam the sensitivity of the spectrometer tends to be rather low since only a small proportion of the total number of ions in the beam can be utilised for analysis. This can be particularly problematical if the system is being used to analyse samples (such as biological or biochemical samples) that are only available in relatively small volumes, especially when such samples are delivered over a relatively short time scale (typically of the order of a few seconds) using a conventional inlet system, such as a liquid chromatograph.
With a view to alleviating this problem, a technique described by R. Grux et al in Int. J. Mass Spectrom Ion.Proc.93(1989) p.323-330 involves using an electron impact ion source to produce ions by electron bombardment, storing the ions for a substantial period of time in a confined space defined by a potential well, and then extracting the stored ions by applying an accelerating voltage thereto whereby to form a burst of ions of relatively short duration. In this way, it is possible to utilise a relatively high proportion of the total number of available ions.
However, this technique suffers from several drawbacks. The technique requires an electron-impact type ion source, and this may be unsuitable for many applications. The ions are subjected to space-charge effects in the confined space and this limits the number of ions that can be stored. Also, the ions tend to oscillate in the confined space and so they have a finite `turn-around` time which limits the minimum duration of each ion burst.